In several technical fields, force sensors and particularly piezoelectric force sensors are employed which comprise at least one piezoelectric body. Such piezoelectric force sensors are commercially available in different designs for a component for measuring forces in one spatial orientation (Fz) up to six components for the measurement of forces and moments (Fx, Fy, Fz, Mx, My, Mz). The fields of application are force measurements in assembly technique, for example in spot welding and in pressing, in the measurement of cutting and forming forces as well as in the field of determining the braking forces in rail vehicles.
Known piezoelectric force sensors that can be employed for the determination of torque or of accelerating forces in addition to force measurement are referred to as piezo load washers. Such a piezo load washer 1 known from the prior art for the measurement of dynamic and quasi-static forces Fz in one spatial direction (z direction) is shown in FIGS. 1a and 1b and will be explained in more detail hereafter. Piezo load washer 1 comprises a multi-part housing 10 on which a connection device 13 is fastened or molded. Typically, the housing 10 is made of steel and has a lid and a bottom. Within the housing 10 is provided a central opening 100 (FIG. 1a) through which a preloading screw (not shown) can be passed. Here, as shown in FIG. 1a, two piezoelectric bodies 11, 11′ having an annular shape surrounding a central axis extending in the z direction are arranged in the housing 10 stacked one on top of the other. Between the piezoelectric bodies 11, 11′ each of which has the shape of an annular body is an electrode 12. The electrode 12 has an electrode body 120 in the shape of an annular body surrounding the central axis extending in the z direction and to which an electrode tab 121 is molded or welded leading away in radial direction from the central z axis. A joining plate 122 curved in the direction of the z axis is welded to the electrode tab 121.
In the circumferential wall of the housing a connection opening 101 is formed leading into a feedthrough 130 of the connection device 13 as shown in FIG. 1a. The electrode tab 121 and the joining plate 122 extend through the connection opening 101 into the interior of the feedthrough 130 up to a contact pin 131 of the connection device 13. Contact pin 131 is electrically insulated from the walls of the feedthrough 130 by means of the insulation 132 and the housing 10 assembled attached thereto. Contact pin 131 is connected to the electrode tab 121 via joining plate 122 in an electrically conductive and form-fit manner.
For the external cable connection of piezo load washer 1 to measurement electronics, connecting cables in the form of highly insulating low capacity coaxial cables are used. These connecting cables generate only minimal frictional electricity upon minimal movement. Such connecting cables for industrial use are commercially available and can be easily connected to the connection device 13 of the piezo load washer 1 so that charge signals can be picked up from the contact pin 131. For the piezo load washer 1 to be able to measure forces the components must be arranged in the housing 10 in z direction in a way that they are pressed onto each other and, thus, the components must be welded immovably fixed inside the housing.
To carry out measurements, the piezo load washer 1 is mounted by means of a preloading screw between two coplanar surfaces of a machine structure under a preload. In case of a load in z direction, piezo load washer 1 emits electrical charges proportional to the applied force Fz. Due to a force acting on the piezoelectric body when a load is applied and by utilizing the piezoelectric effect, charge signals will be generated that are led via electrode 12 across the feedthrough 130 to the contact pin 131 where the charge signals can be picked up and processed from the exterior of the piezoelectric force sensor 1.
Previously, point welding by means of resistance or laser welding has been carried out to achieve an electrical connection of the electrode 12 via electrode tab 121 and joining plate 122 on the contact pin 131. As shown in FIG. 1c, a first piezoelectric body 11′ and the electrode 12 are positioned in the bottom of the opened housing 10 wherein feedthrough 130 has already been fastened to a circumferential surface of the housing. Electrode tab 121 together with the joining plate 122 fastened thereto are arranged such that they extend out of the housing 10 via the feedthrough 130 and such that the joining plate 122 is positioned in front of a front face of the contact pin 131. Then, a welding electrode S can be inserted through connection opening 101 so that the joining plate 122 can be point welded to the front face of the contact pin 131 whereby the electrode 12 is connected to the contact pin 131 in a non-detachable and form-fit manner.
Since the piezo load washers 1 are fabricated with diameters of several millimeters up to several centimeters, it is relatively difficult to perform the point welding on the opened housing 10. Today, due to the miniature format and the different variations in size of available piezo load washers 1, an automation of the fabrication of the electrical connection of electrode 12 to contact pin 131 of the connection device 13 is impossible.
During resistance welding, contaminations occur frequently that interfere with the electrical conductivity of the connection between electrode 12 and contact pin 131. In addition to an insufficient electrical contact between electrode 12 and contact pin 131, short circuits may occur during spot welding. If such poor electrical connections are not detected before the housing 10 is closed and sealed by welding in a pressure-tight manner, the piezo load washer 1 will be completely unusable. Generally, a close examination of the welding after the electrical connection is fed through and prior to closing the housing 10 must be carried out manually, leading to additional effort.